Fix AssertionError in CodegenUtil.getDelegates

Change getAllOverriddenDescriptors contracti, now it returns
original (not substituted) descriptors without any duplicates.

First of all it's necessary in CodegenUtil.getDelegates to avoid
duplicates (see assertion there), but also it's convenient for all
other usages of this method

 #KT-8154 Fixed

compiler/testData/diagnostics/tests/delegation/kt8154.kt

@@ -0,0 +1,11 @@
+interface A<T> {
+    fun foo()
+}
+
+interface B<T> : A<T> {
+    fun bar()
+}
+
+class BImpl<T>(a: A<T>) : B<T>, A<T> by a {
+    override fun bar() { throw UnsupportedOperationException() }
+}

compiler/testData/diagnostics/tests/delegation/kt8154.txt

@@ -0,0 +1,25 @@
+package
+
+public interface A</*0*/ T> {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface B</*0*/ T> : A<T> {
+    public abstract fun bar(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class BImpl</*0*/ T> : B<T>, A<T> {
+    public constructor BImpl</*0*/ T>(/*0*/ a: A<T>)
+    public open override /*1*/ fun bar(): kotlin.Unit
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*delegation*/ fun foo(): kotlin.Unit
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}